1. Field of the Invention
This invention relates to acoustic logging tools for use in a borehole and more particularly to a tool designed specifically for use in conjunction with vertical seismic profiling.
2. Discussion of the Prior Art
Vertical seismic profiling (VSP) is a special technique that is used in seismic exploration. In conventional seismic work on land, a plurality of seismic sensors or geophones are distributed at designated stations along a line of survey in a substantially horizontal plane at or near the surface of the earth. The respective sensor or sensor arrays are connected to a set of individual amplifier channels, such as 96, wherein the sensor signals are processed and recorded. An acoustic source in the vicinity of the line of sensors generates seismic waves. The seismic waves propagate downwardly and become reflected from various subsurface earth layers. The reflected seismic waves return to the surface where they are detected by the sensors as useful seismic signals.
The sensors are sensitive to any slight earth motion, usually the vertical component thereof. For most conventional exploration projects, the amplitude of the geophone signal is proportional to particle velocity although accelerometers sometimes may be used. The useful reflected seismic signals may be expected to be contaminated with noise from various sources such as wind, traffic, footfalls of man or beast, earth unrest and the like. Further, the useful signals may become distorted during their travel through the unconsolidated or weathered earth layer that nearly always overlies the more competent earth formations that lie below. Another source of noise arises from the scattering of the acoustic signals due to small inhomogeneities at or near the earth's surface or to heterogeneities in elastic constants of the layers themselves.
In VSP operations, a seismic sensor such as a geophone or a short array of geophones is successively made to occupy designated stations in a vertical line underneath the surface of the earth. A plurality of acoustic sources are distributed at or near the surface of the earth along a line of survey in a substantially horizontal plane. At each selected depth station the respective sources are triggered in turn, to generate a seismic wave. The respective seismic wave of course, is reflected from subsurface earth layers and is received by the sensor in the borehole. Because the sensor or sensor array may be placed below the earth layers containing the signal scatterers or noise generators, the sensor reposes in a quiet environment, resulting in an improved signal-to-noise ratio. The main advantage however, of a VSP survey is the capability for the direct identification of the propagation modes of seismic signals within the earth. That capability provides us with a better understanding of primary and multiple events as seen from a conventional surface survey; it permits us to more positively separate primary and multiple events for the purpose of multiple suppression. In areas of complex structure and topography, the lateral continuity of desired signals is more easily established. Finally the attenuation of elastic waves in earth materials may be studied. Additional information regarding VSP methods may be gleaned from a paper entitled "The Vertical Array in Reflection Seismology--Some Experimental Studies" by Paul C. Wuenschel published in Geophysics V. 41, No. 2, pp. 219-232. Another useful study is "Vertical Seismic Profiling" by Gal'perin, published by the Society of Exploration Geophysicists, Tulsa, OK.
While it is true that in the conduct of VSP, the seismic sensor may be removed from surface-noise contamination, there are other noise sources that are peculiar to VSP. Some undesired wave forms include tube waves, signals transmitted through the cable supporting the geophone array, multiple reflected waves generated at the bottom of the borehole, and spurious signals transmitted through the drilling fluid in the hole. I have found that tube waves are particularly troublesome. The velocity contrast between the drilling fluid in the borehole and the rock formations that form the borehole wall is sufficiently large that the borehole acts like an acoustic waveguide. A tube wave therefore propagates along the borehole virtually unattenuated. As a noise source, tube waves seriously contaminate the desired reflected waves.
Typically, for VSP work, the sensor or sensor array is mounted in a tool or sonde. The sonde may be lowered into a borehole from a suitable cable by a winch located at the mouth of the hole. The cable may include a stress member and various conductors for reception of seismic sensor signals, transmission of control signals and the like. Most downhole tools include means for locking and unlocking the tool relative to the sidewall of the hole. See for example U.S. Pat. Nos. 2,846,662; 4,365,668; or the previously cited paper by Wuenschel. Usually, the tool contains electronic circuitry, power supplies, and locking control mechanisms as well as a heavy pressure-proof housing so that the tool is quite heavy--200 pounds or more.
It is well known that rigid coupling of the borehole tool to the sidewall of the hole is essential. For example, see Gal'perin, p. 24. The borehole tool is suspended vertically, parallel to the sidewall of the hole. The vertical component of earth motion is transferred to the tool and its contained sensor by a tangential force caused by the propagating body wave. The frictional force between the sidewall and the tool determines the efficiency of transfer of the body waves to the tool and hence the amplitude of the resulting seismic signals. The minimal required frictional force F to prevent sliding of the borehole tool is equal to m*a where "m" is the tool mass and "a" is the acceleration of the borehole wall. If F&lt;m*a, the tool creeps. Since F is proportional to the clamping force and the coefficient of friction and "a" is proportional to signal frequency, we see that creep results in loss of high-frequency signals, generates frictional noise, and allows detection of undesired tube waves.
Earlier workers in the art such as Gal'perin, recommend that the ratio of clamping force relative to borehole tool mass should be greater than unity and preferably up to three times the mass of the tool. Thus for a 200-pound tool, a clamping force of at least 600 pounds is needed. At the same time, once the tool is clamped, the hoisting cable can be slaced off to eliminate cable waves.
I have found that the influence of tube waves is reduced in direct proportion to an increase in the ratio between the clamping force and the weight of the tool. However, there is a practical safe limit to the magnitude of the clamping force that can be applied. Too much force will cause the borehole sidewall to spall if the hole is uncased and such force is capable of rupturing the liner if the hole is cased.
I have also found that the required clamping force for the sensor varies as a function of the type of formations encountered and the resulting local coefficient of friction. A low clamping force/mass ratio that may prevent static creep is not at all adequate to prevent creep under the dynamic conditions of signal propagation, particularly at high frequencies. The clamping-force/mass ratio must be substantially greater than unity; that is, the ratio should be 10:1 or more.